In recent years, visible light-emitting diodes have for example been produced based on vertical InGaN/GaN nanowires containing a p-n junction and connected collectively in parallel.
By virtue of their potential intrinsic properties (good crystalline quality, relaxation of the constraints on the vertical free surfaces, good light extraction efficiency, etc), the nanowires are considered as very interesting candidates for mitigating the difficulties currently encountered with conventional GaN LEDs manufactured in planar (2D) structure).
Two nanowire LED approaches, based on different growth techniques, are known to those skilled in the art.
The first approach consists in epitaxiating GaN nanowires containing InGaN quantum wells in axial configuration by molecular beam epitaxy (MBE). The devices fabricated from these nanowires have given interesting results in the green spectral range. The processed chips of 1 mm2 can emit around 10 μW at 550 nm for a direct operating current of 100 mA.
With the molecular beam (MBE) growth technique, some inhomogeneities appear because of random nucleation mechanisms, but typically an optical power of 50 nW has been obtained on a single wire emitting at 550 nm, i.e. 5 mW/mm2 with a hundred thousand or so emitting nanowires/mm2.
More recently, the MOCVD (metal organic chemical vapor deposition) growth technique has made it possible to produce InGaN/GaN nanowires containing a radial LED structure (core/shell configuration).
FIG. 1 illustrates this type of configuration in which nanowires NTn are produced on the surface of a substrate 20 covered by a nucleation layer 21 making it possible to produce the mesh adaptation between, for example, a substrate in silicon and nanowires in GaN.
The structure of the nanowires exhibiting a photoconductive part, comprising a core 22 in n-doped GaN typically with a doping rate of 1019 cm−3, a quantum well structure with an alternation of layers 23, 24, that can respectively be in non-doped GaN and InGaN, and finally a p-doped GaN layer 25 with typically a doping rate of 1019cm−3. An insulating dielectric layer 26 is provided in order to insulate the core 22 and the upper contact. The upper contacts are ensured via a conductive upper layer 27 transparent to the emission wavelength of the photoconductive structure.
In this approach, the LED structure being in core/shell configuration, the surface of the active zone is greater than in the 2D nanowire LED approach.
This property provides two advantages: the increase of the emissive surface and the reduction of the current densities in the active zone. Complete structures of MOCVD nanowire LEDs have been produced on silicon substrate and the electroluminescence in the blue spectral domain (450 nm) has been obtained on a set of integrated nanowires after technological process.
By virtue of the nanowire growth technologies, the surface zone of a chip can have hundreds of thousands of columns on a surface zone that can typically be 1 mm2.
Such original structures, which exploit nanotechnologies, offer the advantage of increasing the emission surface zone and therefore the light flux.